Systems and methods for vertebral disc replacement

ABSTRACT

The present invention provides artificial disc prostheses, methods and instrumentation for implantation and revision thereof. Each prosthesis may comprise superior and inferior end plates and a nucleus positioned between articular surfaces of the end plates. The end plates may have planar bone engagement surfaces with a plurality of self-cutting teeth. The articular surfaces of the end plates may be planar or include a flattened portion. The nucleus includes superior and inferior articular surfaces which may comprise flattened portions such that when the articular surfaces of the nucleus and the end plates are placed in cooperation in a preferred orientation, the flattened and/or planar portions are aligned. Each prosthesis may provide flexion/extension, anterior/posterior translation, lateral bending, and/or axial rotation degrees of freedom. One embodiment comprises a prosthesis with a first joint providing flexion/extension and anterior/posterior translation, and a second joint providing lateral bending and axial rotation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of:

pending prior U.S. patent application Ser. No. 12/323,068 filed Nov. 25,2008 and entitled SYSTEMS AND METHODS FOR VERTEBRAL DISC REPLACEMENT(Attorney's Docket No. SYD-3), which is a continuation-in-part of:

pending prior U.S. patent application Ser. No. 12/258,961 filed Oct. 27,2008 and entitled SYSTEMS AND METHODS FOR VERTEBRAL DISC REPLACEMENT(Attorney's Docket No. SYD-1), which is a continuation-in-part-of:

pending prior U.S. patent application Ser. No. 12/041,910 filed Mar. 4,2008 and entitled JOINT PROSTHESES (Attorney's Docket No. SYD-4), whichis a continuation-in-part-of:

prior U.S. patent application Ser. No. 11/559,215, filed Nov. 13, 2006and entitled ARTIFICIAL SPINAL DISC (Attorney's Docket No.HO-P03203US2), now patented as U.S. Pat. No. 7,927,374, which is acontinuation-in-part of:

prior U.S. patent application Ser. No. 11/534,014, filed Sep. 21, 2006and entitled ARTIFICIAL SPINAL DISC (Attorney's Docket No.HO-P03203US1), now patented as U.S. Pat. No. 8,172,904, which is acontinuation-in-part of:

prior U.S. patent application Ser. No. 10/590,139 filed Feb. 11, 2008and entitled ARTIFICIAL SPINAL DISC (Attorney's Docket No.HO-P03203USO), now patented as U.S. Pat. No. 8,100,974, filed as a U.S.national stage filing of:

PCT Application No. PCT/US05/023134, filed Jun. 30, 2005 and entitledARTIFICIAL SPINAL DISC, which claims the benefit of:

prior U.S. Provisional Patent Application Ser. No. 60/658,161, filedMar. 4, 2005 and entitled ARTIFICIAL SPINAL DISC, and

prior U.S. Provisional Patent Application Ser. No. 60/584,240, filedJun. 30, 2004 and entitled ARTIFICIAL DISK FOR DEFORMITY CORRECTION.

U.S. patent application Ser. No. 12/258,961 filed Oct. 27, 2008 andentitled SYSTEMS AND METHODS FOR VERTEBRAL DISC REPLACEMENT (Attorney'sDocket No. SYD-1) claims the benefit of:

Provisional U.S. Patent Application Ser. No. 60/982,627, filed Oct. 25,2007 and entitled ALTERNATE ARTICULATION SURFACE ARTIFICIAL CERVICALDISC (Attorney's Docket No. SYD-01 PROV),

Provisional U.S. Patent Application Ser. No. 60/983,500, filed Oct. 29,2007 and entitled ALTERNATE ARTICULATION SURFACE ARTIFICIAL CERVICALDISC (Attorney's Docket No. SYD-02 PROV),

Provisional U.S. Patent Application Ser. No. 61/023,019, filed Jan. 23,2008 and entitled VERTEBRAL DISC REPLACEMENT INSTRUMENTS AND PROCEDURE(Attorney's Docket No. SYD-3 PROV),

Provisional U.S. Patent Application Ser. No. 61/041,086, filed Mar. 31,2008 and entitled VERTEBRAL DISC REPLACEMENT INSTRUMENTS AND PROCEDURE(Attorney's Docket No. SYD-5 PROV),

Provisional U.S. Patent Application Ser. No. 61/050,531, filed May 5,2008 and entitled ARTIFICIAL DISC INSTRUMENTS AND METHODS (Attorney'sDocket No. SYD-6 PROV), and

Provisional U.S. Patent Application Ser. No. 61/074,498, filed Jun. 20,2008 and entitled COMPLIANT PROSTHESIS FOR BALANCE CONTROL ARTHROPLASTY(Attorney's Docket No. SYD-7 PROV).

Pending prior application U.S. patent application Ser. No. 12/323,068filed Nov. 25, 2008 and entitled SYSTEMS AND METHODS FOR VERTEBRAL DISCREPLACEMENT (Attorney's Docket No. SYD-3) is also a continuation-in-partof:

prior U.S. patent application Ser. No. 12/258,977 filed Oct. 27, 2008and entitled SYSTEMS AND METHODS FOR VERTEBRAL DISC REPLACEMENT(Attorney's Docket No. SYD-2), now patented as U.S. Pat. No. 8,454,699.

The above-identified documents are hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to orthopedic medicine, and morespecifically to methods and devices for the treatment of disc diseaseand spinal deformities with artificial disc replacement.

BACKGROUND OF THE INVENTION

Spinal arthroplasty is an emerging field that offers the promise ofrestoring and/or maintaining normal spinal motion. The goal of spinalarthroplasty is to reduce or eliminate adjacent segment disease (ASD) bymaintaining the normal spinal biomechanics at the operative level. Toaccomplish this, an artificial cervical prosthesis must duplicate asclosely as possible the natural spinal biomechanics, includingmaintaining the axial height of the disc as well as applying angularadjustment throughout the full range of motion of the natural spine.

The spine plays an integral role in neural protection, load bearing andmotion. The vertebral column provides a strong, yet mobile central axisfor the skeleton and is composed of twenty-four vertebral bodies withseventy-five stable articulations. The intervertebral disc is afundamental component of the spinal motion segment, providing cushioningand flexibility. Adjacent vertebrae are linked together by threearticulations: a) the vertebral bodies and disc, which transmitcompressive and shear loads and provide flexibility, and b) by two facetjoints, which protect the disc from translational shear stress and limitrotation. This “triple joint complex” allows for flexion, extension,lateral bending and rotation of the spine.

The intervertebral disc is composed of an inner gel-like matrix calledthe nucleus pulposus and an outer surrounding fibrous band called theannulus fibrosus. When compressive loads are placed on the spine,increased pressure in the nucleus pulposus is transmitted to theannulus, which bulges outwards. The degenerative cascade of theintervertebral disc initially involves desiccation of the nucleuspulposus. With decreased elasticity and dampening from the nucleus,increased loads are transmitted to the annulus and facets. The increasedstress on the annulus can lead to fissures and radial tears in itscollagen fibers. With further degeneration, this can lead tocircumferential bulging of the disc, contained and uncontained discherniations, and complete desiccation of the disc. This degenerativecascade can result in axial pain, by stimulating pain fibers in theannulus, or compression of spinal nerve roots and/or the spinal cord.This can manifest itself in motor weakness, pain and/or numbness in thearms or legs or both.

The structure and function of the discs may be altered by a variety offactors including repeated stress, trauma, infection, neoplasm,deformity, segmental instability and inflammatory conditions.Degeneration of the intervertebral disc is the most common etiology ofclinical symptoms referable to the spine. Degeneration of the spine is auniversal concomitant of human aging. In the cervical spine, neck andarm pain caused by nerve root compression has been estimated to affect51% of the adult population. Spondylosis of the spine and aging areintimately related, with spondylosis increasing in both prevalence andseverity with age. Fortunately, the majority of patients will improvewithout surgery. In approximately 10-15% of cases, spondylosis isassociated with persistent nerve root and spinal cord compression and/orspinal pain, with a small percentage ultimately requiring surgery.

The most common type of surgery used in the United States for thetreatment of degenerative disorders of the spine (spondylosis) is spinalfusion. In an interbody fusion, the diseased disc is removed and eithera wedge of bone from the patient's hip, allograft or a metallic spaceris placed between the vertebrae where the disc was removed. Thisimmobilizes the functional spinal unit. While this surgery has beensuccessful in eliminating motion, there are disadvantages associatedwith it. By converting a mobile, functional spinal unit into a fixed,nonfunctional one, fusion results in increased strain patterns at levelsadjacent to the fused segment. When a segment of the spine is fused,there is elimination of motion at the level of surgery. Therefore, thestresses that would normally be absorbed by the disc at the site ofsurgery are now transferred to adjacent segments. This can causeadjacent segment disease (ASD) to one or several spinal units adjacentto the affected level. ASD can be defined as a clinical syndrome ofsymptomatic degenerative changes occurring adjacent to a previouslyfused motion segment. Retrospective studies have estimated that ASD canoccur in the cervical spine at a rate as high as 2.9% per year with aprojected survivorship rate of 26% at 10 years (Hilibrand A S, Carlson GD, Palumbo M, Jones P K, Bohlman H H: Radiculopathy and myelopathy atsegments adjacent to the site of a previous anterior cervicalarthrodesis. J Bone Joint Surg (Am) 81:519-528, 1999).

In the cervical spine, thousands of North Americans undergo surgery forcervical spondylosis each year. The majority of these procedures involvean anterior discectomy with decompression of the spinal cord and/ornerve root. The primary indication for surgery in the management ofcervical spondylosis is radiculopathy, myelopathy and/or neck pain.Following the discectomy, an anterior interbody fusion is commonlyperformed. Autologous bone harvested from the iliac crest or cadavericbone is most commonly used to fill the space created by the removal ofthe disc. A number of other solutions have been suggested, includingmetallic devices such as fusion cages or other types of spacers,xenografts such as bovine bone, and biological strategies such as theuse of growth factors. The graft for the interbody fusion can be shapedto correct underlying deformity of the cervical spine. By contouring thegraft one can restore lordosis to a straight or kyphotic spine.

A more recent alternative to spinal fusion is replacement of the damageddisc with a motion preservation device, which includes either a nucleusor total disc replacement (TDR). The rationale for the development ofthe artificial disc is to prevent adjacent segment disease. Artificialdisc devices can be broadly divided into two categories, those thatreplace the nucleus only, leaving the annulus and vertebral body endplates intact and those that involve replacement of the disc andaddition of prosthetic end plates. Both strategies are directed atrestoration of intervertebral disc function. Prosthetic nuclei aredescribed, for example, in U.S. Pat. Nos. 5,047,055 and 5,192,326.United States Patent application US2002/0183848 also discloses aprosthetic spinal disc nucleus that has a hydrogel core surrounded by aconstraining jacket.

There are several different types of prosthetic devices for use in thecervical or lumbar segments of the spine designed for TDR. For example,the Prodisc™ and the Charite™ disc are composites of cobalt chromium endplates with a polyethylene core. The Prodisc™ is described in U.S. Pat.No. 5,314,477 and the Charite™ disc is described in U.S. Pat. Nos.5,401,269 and 5,556,431. The Prestige™ disc is another type ofartificial disc that comprises a metal on metal design with a ball andtrough articulation. Another type of artificial disc that is gainingpopularity in the cervical spine is the Bryan® disc, described inseveral United States Patent applications including 2004/0098131;2004/00544411; and 2002/0128715. The Bryan® disc is a compositeartificial disc with a low friction, wear resistant, elastic nucleusthat articulates with two circular metal plates.

Presently, there are at least four artificial cervical disc replacementsystems undergoing clinical trials worldwide. These includeunconstrained devices, such as the PCM cervical disc. Theseunconstrained devices do not have mechanical stops to limit their rangeof motion. The Bryan® Cervical disc, the Prodisc™ C and the Prestige™ LPcervical disc systems limit range of motion to varying degrees. Thesesystems can be considered semi-constrained, in that there are mechanicalstops outside the normal range of motion.

Artificial spinal discs have been implanted for the management ofdegenerative disc disease producing radiculopathy, myelopathy and/oraxial spinal pain. More recently, artificial discs have been adopted forthe treatment of trauma. The aim of TDR is to reproduce the biomechanicsof the natural disc. Early clinical and biomechanical studies withsingle and multi-level disc replacement have reported favorable clinicaloutcomes and preserved range of motion at the level of surgery.Preservation of range of motion, however, while an important feature ofan artificial disc, is only a single measure of spinal biomechanics. Theeffect of the disc on angulation at the operative level, the averagedisc space height, and overall spinal alignment (sagittal and coronalbalance) also needs to be considered.

While the introduction of artificial discs has led to many successfulsurgeries, there are still problems associated with the current discs.For example, all of the current artificial cervical discs have a fixedheight across the entire disc. The artificial discs presently availablecan have issues with focal kyphosis or kyphosis at adjacent segments ofthe spine after the patient post-operatively reassumes an uprightposition, supporting the weight of the head and body. For instance, withthe Bryan® disc, the end plates are allowed to move freely about allaxes of rotation, allowing the end plate to assume a position resultingfrom the forces exerted on the implant by the head and neck. At times,this position may be significantly different from the positioning of thedisc intra-operatively. Several published studies with the Bryan®cervical disc replacement system have reported a tendency for the endplates of the prosthesis and the alignment of the cervical spine todevelop kyphosis following surgery. [Pickett G E, Mitsis D K, Sekhon L Het al. Effects of a cervical disc prosthesis on segmental and cervicalspine alignment. Neurosurg Focus 2004; 17(E5):30-35; Johnson J P,Lauryssen C, Cambron H O, et al. Sagittal alignment and the Bryan®cervical disc. Neurosurg Focus 2004; 17(E14):1-4; Sekhon L H S. Cervicalarthroplasty in the management of spondylotic myelopathy: 18 monthresults. Neurosurg Focus 2004; 17(E8):55-61.] This kyphotic angulationof the prosthesis has been attributed to the passive (unconstrainedmotion with a mobile nucleus and variable instantaneous axis ofrotation) design of the implant. None of the current TDR systemsaddresses this major complication.

A significant number of patients with spinal disc disease have a loss ofsagittal alignment of the spine as a result of the degenerative process.In addition, varying degrees of coronal imbalance can also occur. Noneof the available artificial disc replacement systems are designed torestore normal alignment to a spine that is straight, which havefocal/global kyphosis or coronal deformity. Existing artificial discreplacement systems that are inserted into either a straight, kyphoticor angulated segment are likely to take on the angle and localbiomechanics determined by the facets, ligaments and muscle forces. Assuch, patients with a pre-operative straight spine may developpost-operative kyphosis, and patients with a pre-operative kyphosis mayhave a worsening of the deformity post-operatively. Kyphosis of thespine has been implicated in segmental instability and the developmentof clinically significant degenerative disease. Several clinical studieshave described that a change in the sagittal or coronal balance of thespine can result in clinically significant axial spinal pain as well theinitiation and/or the acceleration of ASD. [Kawakami M, Tamaki T,Yoshida M, et al. Axial symptoms and cervical alignment after anteriorspinal fusion for patients with cervical myelopathy. J Spinal Disord1999; 12:50-60; Harrison D D, Harrison D E, Janik T J, et al. Modelingof the sagittal cervical spine as a method to discriminate hypolordosis:results of elliptical and circular modeling in 72 asymptomatic subjects,52 acute neck pain subjects, and 70 chronic neck pain subjects. Spine2004; 29:2485-2492; Katsuura A, Hukuda S, Saruhashi Y, et al. Kyphoticmalalignment after anterior cervical fusion is one of the factorspromoting the degenerative process in adjacent intervertebral levels.Eur Spine J 2001; 10:320-324; Ferch R D, Shad A, Cadoux-Hudson T A,Teddy P J. Anterior correction of cervical kyphotic deformity: effectson myelopathy, neck pain, and sagittal alignment. J Neurosurg 2004;100:S13-S19; Katsuura A, Hukuda S, Imanaka T, Miyamoto K, Kanemoto M.Anterior cervical plate used in degenerative disease can maintaincervical lordosis. J Spinal Disord 1996; 9:470-476.]

Attempting to provide a deformity correction by simply altering the endplate or the nucleus of an artificial disc, while still maintaining freemovement about all axes of rotation, may not be sustainable as theforces exerted by the head and body on the artificial disc couldcounteract the desired correction. To provide a sustainable correction,some limitation on the axes of rotation is required. From a designperspective, the goal is to design an artificial disc that is able tocorrect deformity (coronal and sagittal), has mechanical stops outsidethe normal range of motion (semi-constrained), and preferably hasvariable instantaneous axis of rotation (IAR).

The limits on the axes of rotation can fall into two categories. One isto provide correction using a permanent rotation or translation of anaxis to support the correction. This is accomplished using thegeometries of the core and end plates themselves and is referred to theGeometric Constraint category. The second is to keep free range ofmotion about all axes but provide the correction using a materialsupport. This type of design provides the correction by the impositionof a deformable material in the plane of correction for normal rotationin that plane. This is the Material Constraint category of designs.

Degenerative disc disease is a major source of morbidity in our society.It can lead to serious economic and emotional problems for thoseafflicted. Thus, there is a need for an artificial disc that canalleviate both symptoms and correct deformity (sagittal or coronal orboth) of the spine.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages will be better understood from thefollowing description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention. These and other features of the invention willbecome more apparent from the following description in which referenceis made to the appended drawings wherein:

FIG. 1 illustrates an anterior view of two cervical vertebrae with anartificial disc prosthesis implanted between the vertebrae, theartificial disc prosthesis comprising a superior end plate, a nucleuswhich provides 6° of lordotic correction, and an inferior end plate;

FIG. 2 illustrates an exploded top perspective view of the superior endplate, nucleus, and inferior end plate of FIG. 1;

FIG. 3 illustrates an exploded bottom perspective view of the superiorend plate, nucleus, and inferior end plate of FIG. 1;

FIG. 4A illustrates a top perspective view of the superior end plate ofFIG. 1, and FIG. 4B illustrates a lateral view of the superior end plateof FIG. 1;

FIG. 5A illustrates a top view of the nucleus of FIG. 1, and FIG. 5Billustrates a bottom view of the nucleus of FIG. 1;

FIG. 6 illustrates a lateral cross-sectional view of the nucleus of FIG.1;

FIG. 7A illustrates a posterior cross-sectional view of the nucleus ofFIG. 1, and FIG. 7B illustrates a posterior cross-sectional view of thenucleus of FIG. 1;

FIG. 8 illustrates a lateral view of the artificial disc prosthesis ofFIG. 1 in a preferred orientation;

FIG. 9A illustrates a lateral view of an artificial disc nucleus thatprovides 0° of lordotic correction, FIG. 9B illustrates a lateral viewof an artificial disc nucleus that provides 3° of lordotic correction,and FIG. 9C illustrates a lateral view of an artificial disc nucleusthat provides 6° of lordotic correction;

FIG. 10A illustrates a sagittal cross-sectional view of the artificialdisc prosthesis of FIG. 1 in a preferred orientation orientation in theflexion-extension degree of freedom, and FIG. 10B illustrates a sagittalcross-sectional view of the artificial disc prosthesis of FIG. 1 inextension;

FIG. 11A illustrates a posterior cross-sectional view of the artificialdisc prosthesis of FIG. 1 in a preferred orientation orientation in thelateral bending degree of freedom, and FIG. 11B illustrates a posteriorcross-sectional view of the artificial disc prosthesis of FIG. 1 inlateral bending;

FIG. 12A illustrates a lateral view of the artificial disc prosthesis ofFIG. 1 in flexion and lateral bending, and FIG. 12B illustrates ananterior view of the artificial disc prosthesis of FIG. 1 in flexion andlateral bending;

FIG. 13A illustrates a top view of the nucleus and inferior end plate ofFIG. 1 in a neutral orientation with respect to rotation about acephalad-caudal axis, and FIG. 13B illustrates the a top view of thenucleus and inferior end plate of FIG. 1 in an axially rotatedorientation;

FIG. 14 illustrates an anterior perspective view of an alternativeembodiment of an artificial disc prosthesis comprising a superior endplate, a nucleus, and an inferior end plate;

FIG. 15 illustrates an exploded bottom perspective view of the superiorend plate, nucleus, and inferior end plate of FIG. 14;

FIG. 16 illustrates an exploded top perspective view of the superior endplate, nucleus, and inferior end plate of FIG. 14;

FIG. 17A illustrates a sagittal cross-sectional view of the artificialdisc prosthesis of FIG. 14 in a neutral low-energy orientation withrespect to the flexion-extension degree of freedom, FIG. 17B illustratesa sagittal cross-sectional view of the artificial disc prosthesis ofFIG. 14 in flexion, and FIG. 17C illustrates a sagittal cross-sectionalview of the artificial disc prosthesis of FIG. 14 in extension;

FIG. 18 illustrates an anterior perspective view of an alternativeartificial disc prosthesis comprising a superior end plate, a nucleus, aretention element, and an inferior end plate;

FIG. 19 illustrates an exploded top perspective view of the superior endplate, nucleus, retention element, and inferior end plate of FIG. 18;

FIG. 20 illustrates an exploded bottom perspective view of the superiorend plate, nucleus, retention element, and inferior end plate of FIG.18;

FIG. 21 illustrates a portion of a spine with a partial discectomybetween two cervical vertebrae;

FIG. 22 illustrates a guide tool aligned with the midline of the portionof the spine of FIG. 21;

FIG. 23A illustrates a perspective view of the guide tool of FIG. 22,FIG. 23B illustrates a lateral view of a head of the guide tool of FIG.22, and FIG. 23C illustrates a perspective lateral view of the head;

FIG. 24 illustrates an awl inserted through the guide tool of FIG. 22;

FIG. 25 illustrates a perspective view of the awl of FIG. 24;

FIG. 26 illustrates a cross-sectional sagittal view of the guide tooland spinal portion of FIG. 22, with a driver tool driving a pin into avertebral body;

FIG. 27 illustrates the spinal portion of FIG. 21, with two pins driveninto adjacent vertebral bodies;

FIG. 28 illustrates an adjustable retainer secured to the pins of FIG.27;

FIG. 29A illustrates a rack and pinion system of the adjustable retainerof FIG. 28, with a pawl in a neutral position, FIG. 29B illustrates therack and pinion system with the pawl in a position to allow onlycompression, FIG. 29C illustrates the rack and pinion system with thepawl in a position to allow only distraction;

FIG. 30 illustrates the adjustable retainer of FIG. 28 and a separatedistractor tool;

FIG. 31 illustrates a rasp tool which may be used with the adjustableretainer of FIG. 28;

FIG. 32 illustrates the adjustable retainer of FIG. 28 guiding a planertool;

FIG. 33 illustrates the adjustable retainer of FIG. 28 and a feelertool;

FIG. 34 illustrates a trial;

FIG. 35 illustrates an exploded view of the trial of FIG. 34;

FIG. 36 illustrates the trial of FIG. 34 guided by the adjustableretainer of FIG. 28;

FIG. 37 illustrates an implant inserter gripping the implant of FIG. 1;

FIG. 38A illustrates a lateral view of a distal end of the implantinserter of FIG. 37, FIG. 38B illustrates a top view of the distal endof the implant inserter, FIG. 38C illustrates an end view of the distalend of the implant inserter, and FIG. 38D illustrates a perspective viewof the distal end of the implant inserter in an open configuration;

FIG. 39 illustrates a top cross-sectional view of the distal end of theimplant inserter and implant of FIG. 37;

FIG. 40A illustrates a tamp, and FIG. 40B illustrates an enlarged viewof the distal end of the tamp;

FIG. 41 is a cross-sectional view of the tamp of FIG. 40 fitted to theimplant of FIG. 1;

FIG. 42 illustrates a remover tool gripping the implant of FIG. 1;

FIG. 43 is an enlarged view of a distal end of the remover tool, andimplant of FIG. 42; and

FIG. 44 is an exploded view of the distal end of the remover tool ofFIG. 42.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to systems and methods for the treatmentof disc disease and spinal deformities with an artificial discreplacement. Those of skill in the art will recognize that the followingdescription is merely illustrative of the principles of the invention,which may be applied in various ways to provide many differentalternative embodiments. This description is made for the purpose ofillustrating the general principles of this invention and is not meantto limit the inventive concepts in the appended claims.

In its proper, healthy alignment, the spine follows natural curves,which promote proper sagittal and coronal balance (flexibility) andallow for balanced load sharing between the vertebrae. These curvesinclude the cervical, thoracic, lumbar and sacral regions of the spine.Naturally, in order to accommodate a curve, there must be some variationin the angle of articulation between the functional spinal units and theheight of an intradiscal space. The cervical and lumbar regions arenaturally lordotic, or curved convexly in the anterior direction. Atdifferent segments along the spine, there are typically differentheights for the vertebral bodies and the intradiscal space. In addition,the intradiscal space and vertebral body height may be different fordifferent people.

Each intradiscal space has anterior and posterior regions. An artificialdisc in the cervical, thoracic and lumbar regions that maintains thesame height from the anterior to the posterior may promote an abnormalalignment, resulting in additional stress at the anterior or posteriorportions of an adjacent disc. It may also result in an uneven loaddistribution across the device and cause an excessive amount of relativemotion, wear debris and early failure.

As used herein, the terms, nucleus and core are used interchangeably torefer to an artificial intervertebral device that replaces a damagednatural spinal disc. The artificial core may be provided alone or incombination with a superior end plate for attachment to an uppervertebra or an inferior end plate for attachment to a lower vertebra orboth.

The terms “upper” and “lower” are used herein to refer to the vertebraeon either side of the disc to be replaced, or a surface on a part in theposition shown in the referenced drawing. A “superior” plate is affixedto an upper vertebra and an “inferior” plate is affixed to a lowervertebra of a functional spinal unit.

The terms vertical and horizontal are used herein relative to a standinghuman being in the anatomical position. The term “anterior” refers tothe region towards the front and the term “posterior” refers to theregion towards the back. The term “sagittal” refers to regions on eitherside of the central midline axis of a standing human being. The term“sagittal plane” used herein refers to a vertical plane extending alongthe central midline axis of the vertebral bodies of the spine, dividingthe body into left and right lateral regions. The term “coronal plane”refers to a vertical plane extending along the central midline axis ofthe vertebral bodies of the spine, dividing the body into anterior andposterior regions through the center of the vertebral bodies. The term“cephalad-caudal axis” refers to a vertical axis which extends along thecentral midline axis of the vertebral bodies of the spine.

The term “asymmetrical” is used herein to refer to an axis of maximumheight that is not placed centrally or to a nucleus or total discreplacement (TDR) not having its maximum vertical axis placed centrally.In other words, the maximum height is not situated or pivoted at acenter line of symmetry so that the TDR comprises regions that are notexactly the same in shape or size as other regions on the other side ofa line of symmetry. The location of maximal load bearing is located in anon-central location. The term may analogously apply to joint prosthesesin which an axis of maximum height is not located centrally on asubstantially convex bearing surface, or the axis of maximum depth of adepression is not placed centrally on a substantially concave bearingsurface.

The term “normal alignment” is used herein to refer to the naturalpositioning of functional components of a healthy joint, relative to oneanother and/or the surrounding tissues. Normal alignment may refer tothe static position of a joint at rest, wherein no stress or pressure isplaced on the joint, and it may also refer to the dynamic position of ajoint under natural mechanical stress such as during flexion orextension. Normal alignment may also be referred to as natural, healthy,or proper alignment. “Preferred” or “desired” alignment are used hereinto refer to joint alignment that may be natural, or corrected, butplaces the joint components in a functional or desired position. Theterms “preferred orientation” or “preferred relative orientation” usedherein also refer to component alignment that may be natural, orcorrected, in which the joint components are in a functional or desiredposition.

The phrase “preferred relative orientation” may refer to an orientationabout a single axis, or about multiple axes. For example, an artificialdisc implant may be designed to establish a preferred relativeorientation about an axis extending medial-laterally to provide apreferred anterior-posterior angulation that mimics the appropriatelordosis or kyphosis of the joint motion segment. Alternatively, anartificial disc implant may be designed to establish a preferredrelative orientation about an axis extending generallyanterior-posteriorly to provide a preferred medial-lateral angulationthat provides the desired degree of lateral bending. Such lateralbending may be zero degrees, reflecting the straightness of a healthyspine, or may be nonzero to the left or right to provide correction forvarious pathologies including scoliosis. As another alternative, anartificial disc implant may be designed to provide a preferred relativeorientation about both of the medial-lateral and anterior-posterior axesto encourage proper lordosis or kyphosis while also encouraging thedesired lateral bending. A preferred relative orientation may also be alow energy position in which the joint is naturally encouraged toremain, in contrast to a point of resistance such as a motion stop.

An “orientation feature” is a feature present on one or more jointcomponents that help the components establish a preferred relativeorientation. For example, opposing bearing surfaces on joint componentsmay include flattened sections, which cooperate to urge the componentstoward attaining a preferred relative orientation. Matching curvedsurfaces which align better in a preferred relative orientation may alsobe orientation features. Other configurations of orientation featuresmay be possible in addition to flat and curved surfaces.

It has been found that nucleus body designs with a completely roundedsurface (not necessarily spherical) have issues with reliablymaintaining correction when exposed to the variable forces of the headand neck. To address this issue, one or more segments or sections thatis flat or which has a contour different from the adjacent surface, canbe formed in the nucleus body. This section will be referred to as aflattened section, which is meant to refer to any contour that is notthe same as the adjacent surface(s) of the nucleus. Such a flattenedsurface can be planar or it can have other shapes such as a slightconvex or concave shape with a radius of curvature different from theadjacent surface. Such a flattened surface could also be in the shape ofa compound curve or other complex shape. A flattened section may alsorefer to a rectilinear portion of a two dimensional shape. In theexample of providing a lordotic correction, the flattened segment can beangled relative to the superior end plate of the inferior vertebral bodywith the height of the anterior part being greater than the height ofthe posterior part. The overall shape of the nucleus body is stillasymmetric, but the flattened segment is incorporated to provide areliable correction of the deformity. This flattened segment providesstabilization to resist the moments acting through the nucleus, i.e., ifthe flat is not of adequate size, there may be a tendency for thecorrection to disappear in the presence of an anterior load or for ahyper-lordotic over correction in the presence of a posterior load(during lordotic correction). An additional advantage of incorporating aflattened segment in the nucleus is to provide surface contact over thatarea during small motions about the resting, neutral position of thedevice, which may help reduce the stresses and potentially wear of thedevice.

This flattened surface can be angled relative to the superior end plateof the inferior vertebral body (or vice versa, or both), with the heightof the anterior end being greater than the height of the posterior endwhen lordotic correction is sought. The overall shape of the core canstill be asymmetric, but the flattened surface can be incorporated toprovide a reliable correction of the deformity. Alternatively, the coremay have flattened sections but be symmetric and the endplates may beasymmetric or angled to provide the lordotic correction.

The invention includes a novel artificial disc that provides the normalrange of motion of the natural intervertebral disc, along with theability to correct deformity of the spine. The proposed disc allows forsemi-constrained range of motion of the functional spinal unit. Itreproduces the kinematics of the pre-operative normal spine in allmotions. Of particular, the proposed disc allows for independent &mobile centers of rotation in the flexion-extension and lateral-bendingmotions, which is unique to this device but an inherent characteristicof the natural spine. It possesses maximum durability andbiocompatibility, and a means for integrating itself into the spine bonystructure for long-term stability. Its insertion is safe, simple, andsurgical time is not compromised compared with the current procedures.In contrast to the existing disc replacement systems, it will allow thesurgeon to correct deformity while maintaining natural kinematics of thespine.

In at least one embodiment of the present invention, an artificial disccomprises a nucleus that is not geometrically symmetrical. The disc mayhave a maximum vertical axis that is not located at the geometric centerof the disc. The maximum vertical axis may be located toward the frontof the disc, the rear of the disc and/or on one side of the disc. Thepositioning of the maximum vertical height and load bearing capabilityis chosen depending on the type of deformity that needs to be corrected.The present invention also provides methods for the treatment ofdisc/vertebral body disease, lordosis, kyphosis and scoliosis using anasymmetric artificial disc.

One advantage of the present invention is that the “nucleus” or core maybe interchanged and revised intra-operatively and post-operatively.Instruments can be used to gauge the need for and amount of correctionand the appropriate implant can then be inserted. By introducingcorrection into the nucleus, the surgeon benefits from flexibility, easeof insertion and revisability that present systems do not provide.

Artificial discs of the present invention can be provided with variousdegrees of deformity correction. For this aspect of the invention, thesurgeon can choose a disc having the appropriate correction for thepatient. Thus, a method of treating a spinal deformity is provided. Thismethod comprises preparing a spinal segment for implantation of anartificial disc, determining the desired angle of the intervertebralspace, selecting an artificial nucleus having the desired dimensions,affixing a superior end plate to the upper vertebra, affixing aninferior end plate to the lower vertebra and inserting the selectednucleus between the superior and inferior end plates. Alternatively, andthe assembled unit of end plate-nucleus-end plate may be inserted inunison. The configuration of the nucleus in this pre-assembled constructcan be determined by the intra-operative measurement tools, or withpre-operative calculations. Pre-operative planning techniques andinstruments may also be able to determine the size and orientation ofthis device for insertion.

A major advantage of the present system is that the artificial disc canbe more easily and rapidly inserted and the nucleus can be changed orrevised in accordance with the magnitude of the deformity beingcorrected. This is especially useful in children and young adults wherethe alignment of the spine changes over time.

In at least one embodiment, an asymmetric nucleus adapted for lordoticcorrection of the cervical spine is provided. The surgeon can restorelordosis to the cervical spine while maintaining motion. The nucleus maybe composed of a low friction elastomer such as polyurethane,polycarbonate-polyurethane, a polymer such as polyethylene (particularlyultra-high molecular weight polyethylene or UHMWPE), a suitable ceramic,metals, metal matrix composites such as titanium carbide, or metalalloys such as titanium or a titanium alloy, chrome-cobalt-molybdenum(CoCrMo), cobalt chrome, stainless steel, or other suitable materials.It has a generally trapezoidal geometric design, with varying degrees oflordosis incorporated into it by utilizing an axis of maximum heightanterior to the geometric center of the nucleus. The anterior height ofthe nucleus varies, depending on the extent of lordotic correctionneeded. The nucleus may be available in various lordotic angles, e.g. 0,3° and 6°, as well as differing heights (e.g., 4, 6 and 8 mm).

Before deciding on the final nucleus size, a set of instruments could beinserted to confirm the lordotic correction, but these may also be usedas confirmation for other types of pre-surgical planning techniques andinstrumentation. Alternatively, intra-operative instruments may be usedas confirmation for other types of pre-surgical planning techniques andinstrumentation.

In one embodiment, the implant consists of three pieces; a superior endplate, an inferior end plate, and the nucleus. The end plates will bemade in differing sizes to accommodate differences in anatomy. These maybe fabricated of titanium, titanium carbide, or a titanium alloy,cobalt-chrome-molybdenum (CoCrMo), cobalt chrome, stainless steel, metalmatrix composites, or other materials suitable for spinal prostheticinserts. They may also be mainly fabricated from one or more materialsand utilize a separate coating surface or material layer for optimizingmechanical and wear performance. Coatings could be used for lubricity,low-friction, enhanced hardness, low surface energy, roughness, or otherdesirable characteristics for an articulating joint.

The end plates can have two distinct surfaces. The flat surface of eachend plate, which contacts the vertebral body end plate, is capable ofaccommodating bony ingrowth and incorporates a suitable coating, such asporous titanium, a calcium phosphate, or includes other types of knownsurfaces that promote bony ingrowth for long-term stability. The endplates can also have one or more parasagittal keels or teeth thatprovide immediate fixation.

While the embodiments illustrates below include three piece protheseswith two end plates and a nucleus, it is appreciated that any nucleusdisclosed herein could be integrated with one of the adjoining endplates to provide a two piece embodiment. At least one of the remainingarticular surfaces may be augmented by re-shaping of the surface tocompensate for the motion lost due to integration.

FIG. 1 illustrates an embodiment of an artificial disc replacementimplanted in an intervertebral space between two adjacent vertebrae in aportion of a spine. Artificial disc prosthesis 100 comprises a superiorend plate secured to a superior vertebral body 2, an inferior end plate104 secured to an inferior vertebral body 4, and a nucleus 106positioned between the superior and inferior endplates. Securing an endplate to a vertebral body comprises coupling the end plate to thevertebral body so that it remains in place at least long enough for bonyingrowth to occur. The disc prosthesis 100 comprises a plurality ofarticulating surfaces which form articulating joints, permittingrestoration of intervertebral motion including flexion/extension,anterior/posterior translation, lateral bending and axial rotation,between the end plates. The disc prosthesis 100 further comprisesorientation features which may allow the joints to remain in a preferredorientation relative to one or more axes, which may be a neutral lowenergy position which the joint is naturally encouraged to maintain.

FIGS. 2 and 3 illustrate exploded perspective views of the discreplacement 100; FIG. 2 from a cephalad-lateral perspective and FIG. 3from a caudal-lateral perspective. Various features of the end plates102, 104 are visible in these views. Each end plate 102, 104, isgenerally trapezoidal in shape, although alternative shapes such asrectangular, circular, oval or kidney, among other, are contemplated forother embodiments of the invention. Superior end plate 102 comprises anend plate body 110 with an anterior end 112, posterior end 114, leftlateral side 116, right lateral side 118, superior side 120 and inferiorside 122. The superior side 120 has a bone engagement surface 124 whichis essentially flat, enabling it to easily contact the surface of thenatural vertebral end plate. Use of a flat bone engagement surface mayeliminate extra surgical time needed to prepare the vertebrae to theappropriate shape to accommodate the end plate. However, it isappreciated that other embodiments of the invention may include endplates which are do not have flat bone engagement surfaces, but shapedsurface which may be generally concave or convex. The bone engagementsurface can be porous and incorporate a suitable treatment, such asporous titanium, a calcium phosphate or other types of known treatmentssuch as coatings, plasma sprays, and structural changes to the surface,that promote bony ingrowth or ongrowth for long-term stability. Ananterior portion 126 of the end plate 102 may not incorporate theingrowth treatment, to allow for easier instrument insertion andgripping. A posterior angled portion 128 of the body slopes caudally atan acute angle from the remainder of the body 120, allowing for ease ofinsertion of the prosthesis into the intervertebral space duringimplantation. A plurality of first teeth 130 and second teeth 131 mayproject outwardly from the bone engagement surface 124.

The inferior side 122 of the superior end plate 102 comprises anessentially planar articular surface 132. Two flanges, a left flange 134and a right flange 135 protrude caudally from the articular surface,positioned centrally along the lateral sides 116, 118 of the end plate.The flanges 134, 135 are positioned to fit into gaps formed by notchesformed in the nucleus 106. Other embodiments may include flangespositioned at the corners of the end plates, or at other locations alongthe lateral sides. A sloped surface 136 is formed on the inferior sideof left flange 134, and a sloped surface 137 is formed on the inferiorside of right flange 135. After implantation of the prosthesis andduring lateral bending, the sloped surfaces 136 or 137 do not contact asuperior surface of the inferior end plate, as an inferior surface thenucleus instead contacts the superior surface of the inferior end plateto provide a soft stop to the lateral bending motion. The heights of theflanges 134, 135 may vary, and the slope of the sloped surfaces 136, 137may vary. An anterior-posterior dimension of the flanges 134, 135 may beless than an anterior-posterior dimension of the gaps in the nucleus106, allowing constrained anterior-posterior translation of the endplate 102 relative to the nucleus. A soft stop may occur when a firstcomponent, such as a nucleus, comprising material such as UHMWPEcontacts a second component, such as an end plate, comprising a hardermaterial such as titanium or other metals, in a way as to preventfurther motion of the first component along the same direction.Conversely, the moving first component may comprise the harder material,and the second component may comprise the relatively softer material.

An anterior retention member 140 is formed along the anterior side ofthe end plate 102, protruding caudally toward the inferior end plate104. The anterior retention member 140 may assist in preventingdisplacement of the nucleus from between the end plates, as the member140 is positioned anterior to the anterior edge of the nucleus 106. Aninner edge 142 of the member 140 is angled to permit limited rotation ofthe nucleus relative to the end plate 102 to accommodate device axialrotation. This inner edge 142 also serves as an axial rotation stop tolimit the amount of axial rotation. The inner edge 142 is alsodovetailed to engage gripping arms of a prosthesis insertion tool. Apocket 144 is formed into the anterior portion of the member 140 andserves as a receptacle for instrumentation during implantation, revisionor removal of the prosthesis. After implantation of the prosthesis andduring flexion of the spine, the anterior member 140 does not contactthe inferior end plate 102, as the nucleus contacts the inferior endplate, to provide a motion stop before the member 140 could contact theinferior end plate. Other embodiments of the invention may includemultiple anterior members formed on the superior end plate, or noanterior members formed on the superior endplate.

Inferior end plate 104 comprises an end plate body 150 with an anteriorend 152, posterior end 154, left lateral side 156, right lateral side158, superior side 160 and inferior side 162. An essentially planarsuperior articular surface 164 extends across the end plate body 150. Aposterior retention member 166 is formed at the posterior end 154,protruding from the superior side 160. The posterior retention member isbounded by an inner edge 168 which is angled to permit limited axialrotation of the nucleus 106 relative to the end plate 104, and by asuperior surface 169 which may be angled laterally and posteriorly toallow lateral bending during extension. An anterior retention member 170with an angled inner edge 172 is located along the anterior end 152,protruding from the superior side 160 toward the superior end plate 102.A pocket 174 is formed in the retention member 170, which may receiveinstrumentation during implantation, revision or removal of theprosthesis. A superior surface 176 of the retention member 170 may beangled laterally and anteriorly to permit lateral bending duringflexion.

A pin, or post 180 protrudes from the superior side 160 in a cephaladdirection toward the superior end plate 102. The post 180 may be locatedin a geometric center of the inferior end plate 104, or it may bedisplaced from the geometric center. The location of the post 180, and acorresponding pocket in the nucleus, determines the cephalad-caudal axisabout which the nucleus and the opposing end plate may rotate relativeto the inferior end plate 102. Generally cylindrical in shape to permitrotation about the cephalad-caudal axis, the post 180 comprises acircumferential wall 182 with a spherical shoulder 183, which mayarticulate with a wall of the nucleus pocket. To prevent or limitrotation about a cephalad-caudal axis, the post could have anon-cylindrical shape such as a square or triangle, among others. Thepost 180 also cooperates with the nucleus pocket to permit lateralbending simultaneously with axial rotation.

The inferior side 162 of the inferior end plate 104 comprises a planarbone engagement surface 186, on which one or a plurality of teeth 130,131 may be formed. An anterior portion 188 may be free of bone ingrowthor ongrowth treatments to allow for engagement with instrumentation. Aposterior angled portion 190 of the body slopes cephaladly at an acuteangle from the remainder of the bone engagement surface 186, againallowing for ease of insertion of the prosthesis into the intervertebralspace during implantation.

Referring to FIGS. 4A and 4B, a plurality of bone engagement features,comprising self-cutting first teeth 130 and second teeth 131, may beformed on the bone engagement surfaces 124, 186 of the superior andinferior end plates. Teeth 130, 131 have sharply pointed leading edges,which cut into the vertebral bodies during insertion and may eliminateextra preparation steps such as pre-cutting or reaming grooves into thesurfaces of the vertebral bodies. Each second tooth 131 may bepositioned directly behind, or anterior to, a first tooth 130. Eachfirst tooth 130 has a narrowly angled cutting point 202, and ispositioned with the point oriented toward the posterior end 114 of theendplate 102. As the end plate is inserted between the vertebral bodies,the sharp cutting point 202 on each first tooth 130 cuts a track intothe surface of the vertebral body. As the end plate is slid further in,the second tooth 131 follows in the track cut by the first tooth 130,and a wider cutting point 204 on the second tooth widens the track.

Each first tooth 130 has a pointed apex 206 positioned atop the tooth,supported by a pair of support walls 208 and an end wall 210. Thesupport walls 208 are angled toward one another from the bone engagementsurface 124 or 186 to the apex 206. This angulation advantageouslypermits a solid press-fit as the tooth penetrates the vertebral body,providing immediate anchorage. The angled walls 208 also allow somesubsidence of the end plate 102 into the vertebral body afterimplantation, without the risk of loosening from the vertebral body. Theend wall 210 may be vertical or near vertical, promoting retention inthe vertebral body and prevention of unintended withdrawal from thevertebral body. In other embodiments of the invention, the walls 208 maynot be angled but instead parallel to one another.

Each second tooth 131 comprises the wide cutting point 204, two angledsupport walls 212 and an end wall 216 which support a pointed apex 214.The second teeth 131 are similar in configuration to the first teeth130; however particular dimensions such as wall height and the angle andwidth of the cutting point may vary. For example, as shown in FIG. 4A,second tooth 131 is laterally wider than first tooth 130, while thecutting point 202 of first tooth 130 is narrower than the cutting point204 of second tooth 131. Looking at FIG. 4B, the support walls 212 andapex 214 of the second tooth 131 are taller, providing a larger verticaldimension than the support walls 208 and apex 206 of the first tooth130. A posterior leading edge 218 of first tooth 130 is more gentlysloped than a posterior leading edge 220 of second tooth 131, which mayaid in insertion. All first and second teeth 130, 131 may incorporatethe same bone ingrowth or ongrowth treatments as the bone engagementsurfaces 124, 186.

Both end plates 102, 104 are general laterally symmetrical; however inalternative embodiments one or both of the endplates could have amaximum vertical dimension located on one lateral side to provide ascoliotic correction. Similarly, one or both of the endplates could havean anteriorly located maximum vertical dimension to provide a lordoticcorrection, or a posteriorly located maximum vertical dimension toprovide a kyphotic correction. It is appreciated that specific featuresof the end plates, including but not limited to bone engagementfeatures, motion stops, instrument recesses, and posts, may be swapped,inverted or reversed such that features found on the superior end platemay be instead located on the inferior end plate, and vice versa.Additionally, in alternative embodiments features found on end platesmay instead be located on the nucleus, and vice versa.

FIGS. 5A, 5B, 6, 7A, and 7B illustrate various views of the disc nucleus106. Like the end plates 102, 104, the nucleus has a generally roundedtrapezoidal shape, although alternate embodiments may have other shapes.The nucleus 106 comprises a superior side 250, an inferior side 252, ananterior end 254, a posterior end 256, a left lateral side 258 and aright lateral side 260. The superior side 250 comprises a nuclearsuperior articular surface 262, which further comprises three planarportions: an anterior planar portion 264, a middle planar portion 266,and a posterior planar portion 268. A first curvate transition portion270 lies between the anterior planar portion 264 and the middle planarportion 266, while a second curvate transition portion 272 lies betweenthe middle planar portion 266 and posterior planar portion 268. Thethree planar portions are perpendicular to a sagittal plane of thevertebral bodies when the implant is properly implanted in anintervertebral space. The planar portions are not co-planar with respectto one another, although an alternative embodiment of the inventioncould include one or more co-planar planar portions. An angle a1, theangle between the anterior 264 and middle 266 planar portions, is acuteand may be unequal to an angle a2, the angle between the middle 266 andposterior 268 planar portions. In other embodiments, angles a1 and a2may be equal. The nucleus inferior side 252 comprises an inferiorarticular surface 280, which, like the superior articular surface, alsocomprises three planar surfaces separated by two curvate transitionportions. A right planar portion 284 is separated from a central planarportion 286 by a first curvate transition portion 290, and the centralplanar portion 286 is separated from a left planar portion 288 by asecond curvate transition portion 292. The three planar portions are notco-planar. When the prosthesis is properly implanted in anintervertebral space, the planar portions are perpendicular to a coronalplane of the vertebral bodies when the implant is properly implanted inan intervertebral space. An angle b1, the angle between the right 264and central 286 planar portions, is acute and is equal to an angle b2,the acute angle between the central 286 and left 288 planar portions. Inalternate embodiments, angles b1 and b2 may be unequal to provide ascoliotic correction.

Two notches are formed in the lateral sides of the nucleus, a left notch300 and a right notch 302. The left notch 300 defines a left gap 304,through which left flange 134 extends when the nucleus is positionedbetween the end plates 102, 104. The right notch 302 defines a right gap306, through which the right flange 135 may extend. Each gap 304, 306 iswider in an anterior-posterior dimension than its respective motionstop, to allow translation of the superior end plate 102 relative to thenucleus 106 and the inferior end plate. FIG. 8 is a lateral view of theleft side of the prosthesis, illustrating the relationship of the leftflange 134 to the left notch 300 as the prosthesis is in a low-energyneutral position with respect to rotation about both ananterior-posterior axis and a medial-lateral axis. Arrows indicateanterior-posterior translation of the superior end plate 102. A recess296, visible in FIG. 5A, may be formed in the anterior end 254 towardthe superior side 250 of the nucleus. The recess is shaped to receivethe anterior retention member 140 of the superior end plate 102 duringtranslation of the superior end plate.

Referring to FIG. 5B, a pocket 310 is formed as a recess into theinferior side 252 of the nucleus 106, and is shaped to receive the post180. Pocket 310 is shaped as a tapered slot, with a medial-lateralmaximum opening dimension 312 which is greater than ananterior-posterior maximum opening dimension 314. A support wall 316which circumscribes the pocket 314 tapers outward from an end wall 318of the pocket to its opening 319. The support wall 316 may articulatewith the circumferential wall 182 of the post 180. The ovoid shape andsloping support wall permit the nucleus 106 and the superior end plate102 freedom to move relative to the post 180 during lateral bending.

Prosthesis 100 comprises a combination of articular surfaces and motionstops which allow the flexion-extension rotational degree of freedom andanterior-posterior translation on a first joint and the lateral bendingand axial rotation degrees of freedom on a second joint. The centers ofrotation for each individual rotational degree of freedom may be shared,or each rotational degree of freedom may have a different center ofrotation. The first joint comprises the interface between the nuclearsuperior articulation surface 262 and the inferior articular surface 132of the superior end plate 102. During flexion-extension, the nuclearsuperior articulation surface 262 articulates with the inferiorarticular surface 132. Flexion is limited when the anterior planarportion 264 contacts the inferior articular surface 132, and,conversely, extension is limited when the posterior planar portion 268contacts the inferior articular surface 132. The first joint alsopermits anterior-posterior translation of the superior end plate 102relative to the nucleus 106 and the inferior end plate 104.

The second joint comprises the interface between the nuclear inferiorarticulation surface 280 and the superior articular surface 164 of theinferior end plate 104. During lateral bending, the nuclear inferiorarticulation surface 280 articulates with the superior articular surface164. Left lateral bending motion is limited when the left planar portion288 contacts the superior articular surface 164, and right lateralbending motion is limited when the right planar portion 284 contacts thesuperior articular surface 164. Axial rotation also occurs on the secondjoint, as the nuclear inferior articulation surface 280 rotates relativeto the superior articular surface 164 around the axis of the post 180.This axial rotation motion may be limited by the angled inner edges ofthe anterior 170 and posterior 166 retention members on the inferior endplate 104.

Referring to FIGS. 9A, 9B, and 9C, alternate embodiments of artificialdisc nuclei are illustrated in lateral cross-sectional views. FIG. 9Aillustrates a nucleus 350 in which the upper and lower surfaces 352 and354 are parallel to each other and an angle between them is 0 degrees.In this nucleus, an axis of greatest height 356 falls in the center ofthe disc. In FIG. 9B, a nucleus 360 that provides 3° of lordoticcorrection is illustrated. FIG. 9C illustrates another artificial discnucleus 370 having 6° of lordotic correction. When deformity correctionis provided as shown in FIGS. 9B and 9C, the axis of greatest height 356may shift to a location that is offset from the geometric center of thenucleus. If the anterior/posterior directions are reversed, a kyphoticcorrection is provided. It is apparent that the nucleus can be adjustedto provide various degrees of correction and, in certain cases, if nodegree of correction is needed. Nuclei 350, 360, 370 may be combinedwith end plates 102, 104 or other end plates to form artificial discprostheses. Nucleus 106 comprises 6° of lordotic correction.

The middle planar portions on each nuclear articulation surface mayprovide each joint with a preferred orientation or stable low energyposition. A low energy position is not the same as a motion limitedposition, from which motion of the joint in a particular direction isprohibited after a certain point, i.e. past a motion stop. Instead a lowenergy position is an orientation of a joint into which the joint tendsto settle, and energy must be expended to move the joint out of to thelow energy position. FIG. 10A is a sagittal cross-sectional view ofprosthesis 100 with a joint between the superior end plate 102 and thenucleus 106 in a low energy position in the flexion-extension degree offreedom. Inferior articular surface 132 is in surface contact withmiddle planar portion 266, providing a preferred orientation and lowenergy position of the prosthesis across a coronal plane of the superiorand inferior end plates 102, 104. In order for the joint to move out ofthe low energy position, sufficient energy will have to be applied tothe superior end plate 102 to overcome resistance and rotate anteriorlyacross the coronal plane over the first curvate transition portion 270in flexion or rotate posteriorly over the second curvate transitionportion 272 in extension. FIG. 10B illustrates the prosthesis of 10Awith the joint in extension. The superior end plate 102 is tilted backsuch that inferior articular surface 132 is in contact with posteriorplanar portion 268, the extension motion stop.

FIGS. 11A and 11B illustrate coronal cross-sectional views of prosthesis100 from a posterior perspective, showing the prosthesis in a low energyposition in the lateral bending degree of freedom, and in a laterallybent position, correspondingly. In FIG. 11A, the inferior articularsurface of the nucleus is in a preferred orientation, in unbrokencontact with the superior articular surface 164 of the inferior endplate 104 surrounding the post 180. In order for the prosthesis to moveout of this preferred orientation, sufficient energy will have to beapplied to laterally rotate the nucleus across the sagittal plane enoughto lift one side out of contact with the superior articular surface 164.This position can be seen in FIG. 11B, at the lateral bending stop.FIGS. 12A and 12B illustrates the prosthesis 100 during flexion combinedwith lateral bending. FIG. 12A is a left lateral view, showing superiorend plate 102 tilted forward in flexion, and also shifted forward intranslation, and nucleus 106 is in right lateral bending. FIG. 12B is ananterior view, showing superior end plate 102 in flexion and anteriortranslation and nucleus 106 tilted right in lateral bending. In thisposition, anterior planar portion 264 of the nucleus is in contact withthe inferior articular surface 132 of the superior end plate 102,providing a soft stop to flexion. Right planar portion 284 of thenucleus is in contact with the superior articular surface 164 of theinferior end plate 104, providing a soft stop to lateral bending.

FIGS. 13A and 13B illustrate axial rotation of the nucleus 106 relativeto the inferior end plate 104. For clarity of illustration, superior endplate 102 is not shown. FIG. 13A illustrates nucleus 106 in a central,non-rotated position, and FIG. 13B shows the nucleus 106 rotated to theleft. The angled inner edges 172, 168 of motion stops 170, 166 limit therotational motion. Although not illustrated in these figures, it isappreciated that the present invention allows axial rotation to occur incombination with flexion-extension and/or lateral bending and/oranterior-posterior translation.

It is appreciated that other embodiments of the invention may swap orredistribute the combinations and/or locations of the rotational degreesof freedom. For example, one alternate embodiment may includeflexion-extension and lateral bending in one joint and axial rotation inthe other joint. Another embodiment may include flexion-extension andaxial rotation in one joint, and lateral bending in the other joint.

FIG. 14 illustrates an alternative embodiment of the invention, anartificial disc prosthesis 400 comprising a superior end plate 402, aninferior end plate 404, and a core, or nucleus 406. The superior endplate comprises a superior surface adapted for bony contact and an atleast partially cylindrical surface on an inferior surface to articulatea superior surface of the nucleus. The nucleus includes an at leastpartially cylindrical superior surface and a planar inferior surface anda cylindrical outer surface. The inferior end plate comprises a planarsuperior surface which articulates with the inferior surface of thenucleus, and an inferior surface that is adapted for bony contact and acylindrical inner surface. A first joint of the prosthesis allowsflexion-extension and lateral bending degrees of freedom, and a secondjoint allows the axial rotation degree of freedom.

FIG. 15 illustrates an exploded view of prosthesis 400 from ananterior-inferior perspective, and FIG. 16 illustrates an exploded viewof the prosthesis from a posterior-superior perspective. Superior endplate 402 comprises an anterior end 410, a posterior end 412, a leftlateral side 414 and a right lateral side 416. A gripping recess 417 maybe located on either or both lateral sides. A superior bone engagementsurface 418 is adapted for bony contact, and a plurality of self-cuttingteeth 420 and/or 421 may be distributed on the bone engagement surface418. Both the bone engagement surface and the teeth may incorporate boneingrowth or ongrowth treatments as previously set forth. An inferiorarticular surface 424 comprises a flattened portion shaped as a partialcylinder. A post 430 extends caudally from the inferior articularsurface, comprising post articular surface 432 which may incorporate aflattened section portion. Flattened lateral portions 426, 428, whichmay be planar, flank the inferior articular surface 424.

Inferior end plate 404 comprises an anterior end 440, a posterior end442, a left lateral side 444 and a right lateral side 446. Grippingrecesses 417 may be located on the lateral sides. An inferior boneengagement surface 450, which may be planar, is on an inferior side 448,and may comprise a plurality of self-cutting teeth 420, 421. The teethand bone engagement surface may comprise areas incorporating boneingrowth or bone ongrowth treatments. Referring to FIG. 16, a planarsuperior articular surface 456 is on a superior side 452 of the endplate. An anterior motion stop 460 extends along the anterior end 440and cephalad toward the superior end plate 402. An inner surface 462 iscylindrical to correspond with a cylindrical outer edge of the nucleus406, and an upper surface 464 is shaped as a portion of a cylinder tomate with the inferior articular surface 424 of the superior end plate402 during flexion. A posterior motion stop 466 similarly comprises acylindrical upper surface 468 to mate with the inferior articularsurface 424 during extension. The posterior motion stop 466 furthercomprises a cylindrical inner surface 470, and an undercut 472 which isshaped to receive a tab on the nucleus 406. Right and left lateralmotion stops 474, 476, comprising angled upper surfaces extend towardthe superior end plate 402. Dovetailed inner surface 478, 480 on theright 474 and left 476 stops allow limited axial rotation of the nucleusrelative to the inferior end plate 404.

The nucleus 406 comprises a superior side 490 with a partiallycylindrical superior articular surface 492, and an inferior side 494with a planar inferior articular surface 496. An anterior end 497 iscylindrical to correspond with the anterior motion stop 460 of theinferior end plate 404. A cylindrical posterior end 499 comprises a tab498 shaped to fit in the undercut 472 on the inferior end plate 404. Thetab 498 operates to resist posterior migration (i.e., expulsion) of thenucleus toward the spinal canal by preventing “lift off” of the nucleusfrom the inferior end plate and possible dislocation of the nucleus. Asshown, the tab 498 can be rounded, chamfered or beveled in order tofacilitate initial insertion and intra-operative or post-operativereplacement of the nucleus.

The superior articular surface 492 comprises three flattened sections,each of which is shaped as a portion of a cylinder. Central flattenedsection 500 extends medial-laterally across the nucleus, crossing asagittal plane of the prosthesis. A first curvate transition portion 502lies between the central section 500 and an anterior flattened portion504, while a second curvate transition portion 506 lies between thecentral section 500 and a posterior flattened portion 508. A generallycentrally located pocket 510 extends caudally into the nucleus, and maybe configured to be larger than the post both medial-laterally andantero-posteriorly, to allow limited translation of the superior endplate 402 during flexion-extension and lateral bending. In analternative embodiment of the invention, the pocket 510 may be smallerso that it contacts the post 430 to form motions stops forflexion/extension and/or lateral bending. Another alternative embodimentmay comprise a two piece prosthesis wherein the nucleus 406 is joinedwith the inferior end plate 404, and superior articular surface 492articulates with inferior articular surface 424 of superior end plate402.

FIG. 17A illustrates a sagittal cross-sectional view of prosthesis 400in a preferred orientation position, FIG. 17B illustrates the prosthesisin flexion, and FIG. 17C illustrates the prosthesis in extension.

FIGS. 18-20 illustrate another alternate embodiment of the invention, anartificial disc prosthesis 600. Prosthesis 600 permits the lateralbending degree of freedom on a first upper joint, and theflexion-extension and axial rotation degrees of freedom on a secondlower joint. The prosthesis comprises a superior end plate 602, andinferior end plate 604, and a core or nucleus 606 which is positionedbetween and articulates with the end plates.

FIG. 19 illustrates an exploded view of prosthesis 600 from a superiorperspective, and FIG. 20 illustrates an exploded view from an inferiorlateral perspective. Superior end plate 602 comprises a planar boneengagement surface 608, and a plurality of self-cutting teeth 610, 611may be formed on the bone engagement surface. On an inferior side of theend plate 602 is formed a partially cylindrical inferior articularsurface 612. An anterior retention member 614 is formed on an anteriorend, and has an angled surface 616 which is angled medial-laterally topermit lateral bending during flexion. Gripping slots 618 are formedbehind the retention member 614 as gripping features for instrumentsduring implantation, revision and/or removal of the prosthesis. A pocket620 is formed into the anterior portion of the retention member 614 andserves as a receptacle for instrumentation. A posterior end of the endplate 602 may be slightly angled to assist in insertion into theintervertebral space.

Inferior end plate 604 comprises a planar bone engagement surface 624upon which may be formed a plurality of self-cutting teeth 610, 611.Similar to the superior end plate 602, a posterior end of the end platemay be slightly angled, so that during insertion into the intervertebralspace the posterior ends of the end plates form a reducedcephalad-caudal profile. A superior side of the inferior end plate 604comprises a superior articular surface 626, which in turn comprisesthree flattened portions separated by curvate transition portions. Theflattened portions, anterior planar portion 628, middle planar portion630, and posterior planar portion 632 are not co-planar with respect toone another, and are perpendicular to a sagittal plane of the vertebralbodies when prosthesis 600 is properly implanted in an intervertebralspace. Middle planar portion 630 cooperates with a planar inferiorsurface of the nucleus to provide a neutral low energy position orpreferred orientation of the prosthesis in the flexion-extension degreeof freedom. The anterior planar portion 628 forms a soft motion stopwhen it contacts the planar inferior surface of the nucleus duringflexion, and the posterior planar portion 632 forms a soft motion stopwhen it contacts the planar inferior surface of the nucleus duringextension. A post 634 protrudes from the superior articular surface 626and cooperates with a pocket in the nucleus to permit anterior-posteriortranslation during flexion-extension. An anterior retention member 636protrudes from the end plate 604 toward the opposite end plate 602, anda pocket 621 is formed in the anterior end of the end plate inferior tothe anterior retention member. Left 638 and right 640 lateral motionstops are formed on the lateral edges of the end plate. Each of the leftand right lateral motions stops includes a tri-sloped upper surface, topermit flexion-extension during lateral bending, and vice versa. Insideedges of the anterior 636, left 638 and right 640 motion stops are alsoangled to permit axial rotation of the nucleus about the axis of thepost 634. Alternative embodiments of the inferior end plate 604 mayincorporate a lordotic or kyphotic correction such that a maximumvertical axis of the end plate is displaced anteriorly or posteriorlyfrom the center of the end plate.

The nucleus comprises a planar inferior articular surface 642 and asemi-cylindrical superior articular surface 644. The planar inferiorarticular surface 642 articulates with the superior articular surface626 of the inferior endplate to provide flexion-extension and axialrotation, and the superior articular surface 644 articulates with theinferior articular surface 612 of the superior end plate 602 to providelateral bending. An elongated pocket 646 is recessed into the inferiorside of the nucleus, and is shaped to receive the post 634. A curved tab648 projects posteriorly from the nucleus with an undercut 649 formedunder the tab.

A separately formed retention element 650 which is joined to thesuperior end plate 602 by welding or other means may engage with thenucleus 606 to retain the nucleus 606 in the prosthesis and also toserve as a motion stop. The retention element 650 comprises a body 652,a pair of arms 654 and a projection 656. The projection 656 fits intothe undercut 649 on the nucleus, and the arms 654 fit over the curvedtab 648. During spinal motion, the retention element moves with thesuperior end plate. The inferior surfaces of the body 652 and projection656 are angled to allow both lateral bending and extension.

The intervertebral disc implants depicted in FIGS. 1-20 may be formed ofbiocompatible materials such as bio-compatible metals or other suitablestrong materials. An implant may be formed of one biocompatible materialwhile the bearing surface comprises another biocompatible material. Theimplants may be constructed in a variety of footprint sizes, and avariety of shapes, to fit the variations found in patient vertebralsizes and vertebral shapes. Specifically, each implant may be availablein three footprint sizes: small, medium, and large, and in threecorrective lordotic angles: 0°, 3°, and 6°.

The implant components may be formed wholly or partially of anybiocompatible metal, such as stainless steel, Titanium, Titanium alloys,Cobalt Chrome, CCM (Cobalt Chrome Molybdenum), Aluminum, Zirconium,Nickel Titanium (NiTi/Nitinol), shape memory metals, superelasticmetals, metal matrix composites such as Titanium Carbide, TungstenCarbide, Tantalum, or Chromium, among others. The implant components canbe formed wholly or partially of a biocompatible ceramic material suchas alumina, zirconia, alumina-zirconia blends, or other ceramiccomposites. The implant components can be formed wholly or partially ofa biocompatible polymer such as PEEK, carbon or glass-fiber reinforcedpolymers, ABS, polycarbonate, polyethylenes, ultra high molecular weightpolyethylene (UHMWPE), nylons, polymer composites, polyurethane,polycarbonate-polyurethane composites, acetals, polyesters,polypropylene, PTFE, ePTFE, absorbable polymers such as poly l-lacticacid (PLLA), polylactic acid (PLA), polyglycolic acid (PGA), TCP,glycolides, lactides, hydrogels, elastomers such as silicone, nitrile,butyl, thermoplastic elastomers (TPE's), or ethylene vinyl acetate(EVA), among others.

The implant components can be can be formed wholly or partially ofanother biocompatible material including diamond or diamond-likematerials, carbon, hydrogels, pyrocarbon, pyrolitic carbon, allograftbone, demineralized bone, collagen, cartilage, tricalcium phosphate,calcium phosphate, hydroxyapatite, PMMA—bone cement, proteins, aminoacids, nucleic acids, or sugars, among others.

The implant components may also be coated wholly or partially withspecialized coatings such as Titanium Nitride, Titanium Boride, TitaniumCarbide, ion-based coatings, ceramic coatings, oxide coatings, plasma,PTFE coatings, low-friction coatings, hydrophobic or hydrophiliccoatings, or vapor deposition coatings, among others. Bone-contactingportions of implant components may comprise porous or non-porous boneingrowth surfaces.

In another aspect of the invention, all of the articulating surfaces ofthe prosthesis can be formed of a polymer. As discussed above, thenucleus can be formed entirely of a polymer such as, for example,ultra-high molecular weight polyethylene (“UHMWPE”), a cross, linkedUHMWPE, a ceramic, polyetheretherketone (“PEEK”) or other type ofsuitable polymer. The bony ingrowth surfaces can be made from plasmasprayed metals, hydroxyapatite or similar bone-like coatings, and caninclude a coating of bone growth factors. The articulating surfaces ofthe end plates can be formed with inserts of an appropriate polymer,ceramic or the like. The remaining exterior surfaces of the end platesthat interface with bone can be formed with bony ingrowth surfaces ofthe type discussed above.

FIGS. 21-41 illustrate instrumentation and methods for implanting anartificial disc prosthesis such as prosthesis 100 into a portion of aspine. A kit of tools, including implant trials in a variety of sizes,may be provided. Referring to FIG. 21, the patient is prepared in aneutral posture position, and the target disc level of the spine isexposed from an anterior approach. A partial discectomy is conducted ina targeted intradiscal space between superior vertebral body 2 andinferior vertebral body 4. Although cervical vertebral bodies 2 and 4are depicted in the illustrations as the C4 and C5 level vertebrae, itis appreciated that the procedure could be performed on other vertebralpairs in the spine.

Referring to FIG. 22, a sagittal midline 6, or central midline axis ofthe vertebral bodies is determined and may be marked on the exposedvertebral bodies. A guide tool 700 is preliminarily positioned on thevertebral bodies, aligning sagittal indicators 702, 704, 706 with thesagittal midline 6 when viewed from a viewpoint normal to the coronalplane. Alternatively, a line may be determined which is not on thesagittal midline but is parallel to the sagittal midline, and the guidemay be aligned to this offset line instead.

FIG. 23A illustrates guide tool 700 in its entirety, FIG. 23Billustrates an enlarged side view of a guide head 710, and FIG. 23Cillustrates an enlarged perspective view of the guide head. Guide tool700 comprises a handle 712 which comprises a proximal gripping portion714 and a distal shaft portion 716. The shaft 716 is welded to the head710. The guide head 710 is shaped as an elongated loop and comprises acircumferential wall 718 which defines a guide lumen 719. The head 710has a first side 720 and a parallel second side 722, through which ports724 and lateral alignment holes 726 open. A guide tab 728, whichincludes the sagittal indicator 706, protrudes distally and is connectedto both first 720 and second 722 sides. As seen in FIG. 22, the guidetool 700 may be first positioned such that the guide tab 728 protrudesinto the space created by the partial discectomy and the sagittalindicators are lined up with the sagittal midline 6. The guide tool 700is further manipulated so that the lateral alignment holes 726 on side720 align with the lateral alignment holes 726 on side 722, appearingconcentric with one another when viewed from a viewpoint normal to thesagittal midline. In this way, the guide head is accurately positionedrelative to three orthogonal planes, in a preferred orientation parallelto the sagittal plane and perpendicular to the coronal plane of thevertebral bodies. Fluoroscopy may be used to determine and direct thealignment processes. Another embodiment of the invention may comprise asingle lateral alignment hole 726 on each side 720, 722.

Referring to FIGS. 24 and 25, an awl 750 may be used to create pilotholes in the vertebral bodies. A distal end 752 and tip 754 of the awl750 are shaped to fit through the guide lumen 719. The tip 754 of theawl may be used to penetrate each of the vertebral bodies 2, 4, alongthe midline and approximately mid-body, creating pilot holes for guidepins.

Referring to FIGS. 26 and 27, a guide pin 762 is inserted through theguide lumen 719 of the head 710 and driven into vertebral body 2 on thesagittal midline and a guide pin 764 is driven into the adjacentvertebral body 4 on the sagittal midline, the pins co-planar with oneanother, using the pilot holes if necessary. Guide pins 762 and 764 maybe identical, or may differ in length. Guide pins 762, 764 each comprisea distal threaded penetrating tip 770, a distal shaft portion 772, amiddle shaft portion 774 and a proximal shaft portion 776. A recessedgroove 775 encircles the shaft, providing an interface for connection toother instruments. A driver engagement interface 778 is configured toengage with a corresponding drive feature 782 on a driver tool 780,which is rotated to drive and each pin 762, 764 into its respectivevertebral body. The interface 778 and corresponding drive feature 782may be shaped as a hexagon or another shape. The guide pins 762, 764 areimplanted on the sagittal midline approximately mid-body and parallel tothe target disc space. Once the guide pins are secured, the guide 700tool may be removed.

Referring to FIGS. 28 and 29, a retainer 800 is placed on the pins 762,764. The retainer 800 is an adjustable bracket system which may beplaced in engagement with the pins 762, 764 to adjust and maintain adistance between the pins and the vertebral bodies in which the pins aresecured, thereby providing an accessible working area in theintervertebral space between the targeted vertebral bodies. A rack andpinion system provides compressive force or distractive force to urgethe pins and therefore the vertebral bodies together or apart. Theretainer 800 also provides a guiding framework for additionalinstruments, permitting the instruments and prostheses to be placed in apreferred orientation with respect to the sagittal midline of thevertebral bodies. Referring to FIG. 28, the retainer 800 comprises afirst bracket 802, a second bracket 804, a rack and pinion system 806,and two plates 808, 810 which are configured to fit over and engage thepins 762, 764. Plate 808 may be fit over pin 762 and locked to the pinby engaging a lock 809, and plate 810 may be fit over pin 764 and lockedto the pin by engaging a lock 811. Each lock 809, 811 comprises a tab813 which is rotated in one direction into engagement with the groove775 on the respective pin to provide a locked configuration, and may berotated in the opposite direction to provide an unlocked configuration.A pair of links 812, 814 hingedly connect the plates 808, 810 to thebrackets 802, 804. Between the plate 810 and the second bracket 804 isan additional link 816 and a pivot pin 817, around which plate 810 whichmay be pivoted to allow angular movement of plate 810 and pin 762 duringprosthesis implantation, revision and/or removal procedures. A collar818 is slidable between a first position, seen in FIG. 28, in which itprevents pivoting of plate 810 around the pivot pin, and a secondposition in which plate 810 is free to pivot. When plate 810 is allowedto pivot, the angularity of plate 810, pin 762 and associated vertebralbody 2, may be adjustable out of a parallel position relative to plate808, pin 764 and associated vertebral body 4. This adjustability may beuseful or necessary during the insertion of trials into theintervertebral space, or during other steps of the implantationprocedure. FIGS. 29A-29C illustrate cross-sectional views of thebrackets 802, 804 and the rack and pinion system 806. The rack andpinion system 806 is housed inside a bracket housing 805, although therack may extend out of the housing. The rack and pinion system 806includes a rack 820 with a row of rack teeth 822. The rack 820 isrigidly connected to the first bracket 802 by welding or other means,and passes through the housing 805. A spring-loaded stop arm 821 extendsfrom the rack and prevents the rack from being unintentionally withdrawnfrom the housing 805. When withdrawal of the rack from the housing isdesired, the stop arm 821 may be depressed toward the rack 820,compressing a spring 823. A pinion 824 includes pinion teeth 826 and maybe turned by a pinion wing 828 (seen in FIG. 28). The position of a pawl830 controls whether the retainer provides ratcheting compressive ordistractive force to the pins 762, 764, and the pawl is movable betweena first position, a second position, and a third neutral position. Thepawl 830 comprises a first pawl tooth 832, a second pawl tooth 834, andis pivotable about a pivot pin 836. A toggle 838 may be switched betweenthe first, neutral and second positions, controlling a spring 840 andplunger 842 which engage the pawl 830, moving it between the first,neutral and second positions. FIG. 29A shows the toggle 838 and the pawl830 in the neutral position. In the neutral position, the pawl is notengaged with the rack teeth 822, and the rack 820 can move in eitherdirection relative to the second bracket 804 by turning the pinion 824to engage the rack. To attain the first, or distraction, position, thetoggle 838 is moved in a first direction 850 such that the plunger 842may depress the first pawl tooth 832 into engagement with the rack 820,as shown in FIG. 29C. With the pawl in this first position, the rack 820can only move in direction 852 relative to the second bracket 804 as thepinion 824 is turned to engage the rack. Since the rack 820 is connectedto the first bracket 802, first bracket 802 also moves in direction 852relative to the second bracket 804, moving the brackets 802, 804 awayfrom one another. Since brackets 802, 804 are linked to plates 808, 810locked to the pins 762, 764, movement of the rack in direction 852results in distraction of the pins 762, 764 and the vertebral bodies 2,4.

Referring to FIG. 29B, when the toggle 838 is moved past the neutralposition in a second direction 852, plunger 842 may depress the secondpawl tooth 834 into engagement with the rack 820, placing the pawl 830in a second, or compression, position. With the pawl in this secondposition, the rack 820 can only move in direction 850 relative to thesecond bracket 804 as the pinion 824 is turned to engage the rack,thereby moving brackets 802, 804 closer together. Movement of thebrackets 802, 804 closer together results in compression of the pins762, 764 and the vertebral bodies 2, 4. Before the retainer is placed onthe pins 762, 764, the pawl 830 may be placed in the neutral position,permitting the rack to move freely in either direction, and allowing adistance between the brackets and plates to be adjusted to match adistance between the pins. The plates 810, 808 are place over the pins762, 764 and the locks 809, 811 are engaged to lock the plates to thepins. The driver tool 780 may be used to engage the locks 810, 811.Then, distraction or compression may be accomplished by the methodsdescribed above, i.e., the toggle 838 is moved to the first position andthe pinion is turned to provide only distraction, the toggle 838 ismoved to the second position and the pinion is turned to provide onlycompression. Alternatively, the toggle 838 may be placed in the neutralposition to allow unconstrained distraction and/or compression. It isappreciated that the retainer 800 may be placed over the pins 762, 764in either direction, that is, plate 808 may be place over pin 764 andplate 810 over pin 762, or alternately, plate 810 may be placed over pin764 and plate 808 over pin 762. It is also appreciated that in analternative embodiment of the invention, each bracket may comprise anadjustable feature such as a rack and pinion system, to providedistraction and compression between the brackets, pins and associatedvertebral bodies. In addition, a pivoting feature such as pivot pin 817and collar 818 could be on either or both plates.

As seen in FIG. 28, link 812 connects plate 808 with bracket 802, andlinks 814 and 816 connect plate 810 with bracket 804. The links andbrackets may be hinged so that they may be rotated about thecephalad-caudal axis of the vertebrae, toward one lateral side or theother, allowing for optimal visibility and access to the surgical site.

As seen in FIG. 30, a distractor 900 may be used with the retainer 800to aid in providing sufficient distraction between the vertebral bodies2, 4, if necessary or desired. Prongs 902, 904 of the distractor 900 maybe inserted into the intervertebral space, and levers 906, 908compressed together to provide distraction. A ratcheting mechanism 910allows the levers to be locked in a fixed position, and a leaf spring912 provides the resistance for the distraction. Such distraction may beprovided while the retainer pawl 830 is in the neutral position or inthe first position. Once the vertebral bodies are sufficientlydistracted, the retainer pawl 830 may moved to the first position if notalready there, to maintain the spacing between the vertebral bodies.Distractor 900 may comprise a locking feature to hold the levers 906,908 and prongs 902, 904 in a fixed position, until released.

After the vertebral bodies are sufficiently distracted, a discectomy anddecompression may be performed using instruments know in the art such asronguers, curettes and osteotomes. Bone rongeurs, planers, rasps, buntools or other instruments may be used to prepare flat surfaces on thevertebral endplates, as flat surfaces may ensure the best interfacebetween the prosthesis end plates and the vertebral endplates. Endplatepreparation may also include forming grooves to correspond with teeth orkeels of a prosthesis, roughening or smoothing the surface to enhanceconformance with the prosthesis or encourage bony ingrowth andstabilization of the prosthesis, and/or contouring the shapes of theendplates. FIG. 31 illustrates a rasp 950 which may be inserted betweenthe vertebral bodies 2, 4 to scrape and flatten the endplates. Rasp 950comprises a gripping portion 952, a shaft 954, a pair of wings 956 and arasp head 958. The rasp head comprises a plurality of cutting edges 964,which may be undercut and each of which may be adjacent an opening 965,which during rasping may allow cut material to flow through to theopposite side of the head and not clog the cutting edges. Additionally,the cutting edges may cut only in the posterior-to-anterior direction,making insertion into the intervertebral area easier and less traumaticthan it would be with a rasp which cuts in the anterior-posteriordirection. The wings 956 comprise wing plates 960, 962 which flankeither side of the shaft 954 and are aligned perpendicular to the rasphead 958. The wings 956 are a guiding feature which allow the rasp 950to be inserted into the intervertebral space in alignment with theretainer 800, along a pathway substantially parallel with the pins762,764. With reference to FIG. 28, the rasp 950 may be inserted betweenthe plates 808, 810 such that the wing plates 960, 962 slide overretainer plates 808, 810 in a fixed orientation. With the rasp 950 thusaligned, the rasp head 958 will enter the intervertebral space in anpreferred orientation parallel to the vertebral endplates and relativeto the sagittal plane. It is appreciated that other instrumentsincluding but not limited to a planer, blade, grater, or cutter couldhave a guiding feature comprising similar wing plates, permittingalignment with the retainer plates 808, 810 and correct orientation ofthe instrument relative to the sagittal plane. FIG. 32 illustrates aplaner which may also be used in endplate flattening and preparation inan alternative embodiment. Planar 970 comprises a gripping portion 972,a shaft 974, a pair of wings 976 and a planer head 978 with a cuttingedge 979.

Referring to FIG. 33, a feeler 980 may be employed to evaluate theconfiguration of the intervertebral space, to assess endplate flatnessand determine which implant footprint best fits the space. The feeler980 comprises a handle 982, a shaft 984 and a paddle 986 with flatsides. The feeler may be available in a plurality of sizes such assmall, medium and large, each size comprising a paddle with a comparablefootprint size to a prosthesis such as prosthesis 100. As seen in FIG.33, the feeler may be inserted between the plates 808, 810 of theretainer until the paddle 986 is in the intervertebral space. Visualobservation or fluoroscopy may be used to observe the size of the paddle986 relative to the vertebral endplates, to determine the correctprosthesis footprint size. The paddle 986 may be pressed or rubbedagainst the prepared vertebral endplates to assess flatness of theendplates, and/or fluoroscopy may be used to observe the profiles of theendplates compared to the profile of the paddle to assess flatness. Thefeeler may be available in a variety of sizes, and other embodiments ofthe feeler may include wings such as those on the rasp 950 to allowprecise guidance by the retainer 800. Once the flatness of the vertebralbody endplates is assessed, additional preparation with a rasp, planer,hammer, burr and/or other tools may occur if necessary to relieveconcavities, convexities, or other irregularities on the endplatesurfaces. These steps of assessment and preparation may be repeated asneeded.

Referring to FIGS. 34-36, a trial or trials may be inserted into theprepared intervertebral space to determine the lordotic correction, ifany, that is needed. Trials are available in a variety of footprintsizes, matching the feeler and prosthesis footprint sizes. Alternativeembodiments of trials may include those shaped as intervertebral discreplacements, fusion cages, spacers, or other intervertebral implants.FIG. 34 illustrates a trial 1010, and FIG. 35 illustrates a partiallyexploded view of a distal end of the trial. Trial 1010 comprises a head1000 with first trial plate 1002 and a second trial plate 1004. Trialplate 1002 has a peg 1006 (not visible in FIG. 35) which joins it to afirst lever 1012, and trial plate 1004 has a peg 1007 joining it to asecond lever 1014. The plates 1002, 1004 may be secured to the inserterby inserting each peg through a corresponding hole in the distal end ofeach lever. Alternately, the plates may be permanently welded to thelevers.

Trial 1010 further comprises a first lever 1012 and a second lever 1014.At their proximal ends, the levers 1012, 1014 are joined by a ratchetingmechanism 1016. Near their distal ends, the levers are linked by a rivet1015. Rivet 1015 is joined to lever 1012 and captured in a slot 1017 onlever 1014, such that the levers can move relative to one another butsuch movement is constrained by the length of the slot 1017. First lever1012 comprises a first wing 1018, and second lever 1014 comprises asecond wing 1019, the wings positioned so that the inserter may be slidover the plates of the retainer 800, positioning the inserter withrespect to the pins 762, 764 and the targeted intervertebral space. Apivot pin 1020 joins the levers 1012, 1014 at their distal ends,allowing the levers to rotate about the pin 1020 and pivot relative toone another.

FIG. 36 illustrates insertion of the trial 1010 into the intervertebralspace between the prepared endplates. A trial is chosen with anappropriate footprint size determined by use of the feeler. The proximalends of the levers are positioned so that the distal ends areapproximately parallel to one another, so that the trial plates 1002,1004 are also parallel with respect to one another. The trial 1010 isinserted between the retainer plates 808, 810, and the levers arepositioned so that the wings 1019, 1018 flank the plates 808, 810,thereby positioning the head 1000 in a preferred orientation relative tothe sagittal plane. The head 1000 is further inserted, into theintervertebral space. Fluoroscopy may be used to place the head at adesired depth within the intervertebral space.

The desired degree of lordotic correction may be determined by adjustingthe angle of the trial plates 1002, 1004 within the intervertebralspace. Levers 1012, 1014 are ratcheted together, causing their distalends to pivot apart around the pivot pin 1020, and causing trial plates1002, 1004 to pivot apart until the desired angle, or degree of lordoticcorrection is reached, which may be visualized through fluoroscopy. Areference feature, which may comprise markings and/or alignable holes onthe trial 1010 may also be used to measure the degree of lordoticcorrection. Once the degree of lordotic correction is determined, thetrial inserter 1010 may be released, allowing the trial plates 1002,1004 to return to a parallel position for removal, and the trial 1000 isremoved from the intervertebral space. Observations of footprint sizeand degree of lordotic correction may be used to select a properlyconfigured prosthesis for implantation. Another embodiment of the trialmay include a shaft which is distally displaced to pivot the trialplates.

One reference feature on the trial may comprise holes located on thefirst lever, which may be coaxial with a slot on the second lever toindicate an angulation or degree of lordotic correction. First lever1012 comprises an array of holes 1022, and second lever 1014 comprisesan elongated slot 1024. When the first and second levers are at oneposition relative to one another, and therefore the plates are at oneangle, a first hole in the array 1022 is coaxial with the slot 1024.When the first and second levers are at a second position, and thus theplates at a second angle, two holes in the array 1022 are coaxial withthe slot 1024. When the first and second levers are at a third position,and thus the plates at a third angle, three holes in the array 1022 arecoaxial with the slot 1024. It is appreciated that in other embodimentsof the invention, the array may comprise more or less than three holes,and the array and the slots may be situated at various locations on thetrial. The alignment of the array with the slot may is viewed from aviewpoint normal to the array, and may be viewed unaided or may beviewed through the use of fluoroscopy. Alternatively, or in addition tothe coaxial holes and slot, markings 1026 on the ratcheting mechanismmay indicate the angulation or degree of lordotic correction.

FIGS. 37-41 illustrate the insertion of an intervertebral discprosthesis 100 into the prepared intervertebral space between vertebra 2and 4. It is appreciated that the methods and instrumentation presentedherein could be used to implant prostheses 400, 600 or other similarartificial disc prostheses. Referring to FIG. 37, an implant inserter1050 is shown, gripping the prosthesis 100. Implant inserter comprises arotatable handle portion 1052, a fixed handle portion 1053, a shaft1054, wings 1056, and a gripping mechanism 1058. The shaft 1054comprises an outer tube 1060 and a rod 1062 positioned inside the tube.The outer tube 1060 may comprise a plurality of cleaning slots 1061. Therotatable handle portion 1052 is connected to the rod 1062 such thatturning the rotatable handle portion 1052 moves the rod 1062 distally orproximally. At its distal end, the rod is connected to the grippingmechanism 1058. When the rod 1062 is displaced distally to a firstposition, it cams the gripping mechanism 1058 into an open position, andwhen the rod is displaced proximally to a second position, the grippingmechanism is cammed into a closed position in which the prosthesis 100may be securely gripped for implantation. The wings 1056 are configuredto slide over and flank the retainer plates 808, 810 during implantationto place the instrument and prosthesis in a preferred orientationrelative to the sagittal plane.

Referring to FIGS. 38A-38D, enlarged views of the distal end of theinserter and the gripping mechanism are shown. The gripping mechanismcomprises a first alignment side 1070 and a second alignment side 1072which is positioned opposite the first. The alignment side 1070, 1072are oriented perpendicular to the wings 1056, to place the prosthesis inthe proper orientation during implantation. Each alignment membercomprises a plurality of prongs 1074 which extend past the ends of thealignment sides, and between which the prosthesis is sandwiched whengripped by the gripping mechanism for handling and insertion. The prongsmay be positioned to line up with teeth on the prosthesis end plates102, 104, for ease of insertion. As the prosthesis is mounted to theinserter, the end plates 102, 104 may be compressed together into apreferred orientation to sandwich the prosthesis together and then slidbetween the prongs; this compression may help prevent the intervertebralspace from becoming overstuffed with the insertion of the prosthesis. Atits distal end, which is shaped to complement the anterior end of theprosthesis end plate 102, alignment side 1070 comprises a first key1076, which is shaped to fit coaxially in a pocket 144 on the anteriorend of the end plate 102. Alignment side 1072 comprises a second key1078 larger than the first key, shaped to fit coaxially in a pocket 174on the anterior end of the end plate 104. The keys and pockets arespecifically sized so that the prosthesis can be mounted on the inserter1050 in only one, correct, position. It is appreciated that otherembodiments of the inserter 1050 may include keying features shaped toengage with gripping recesses, pockets, or other features of implants400 or 600.

The gripping mechanism further comprises two pivotable opposing arms1080, 1082. When the inner rod 1062 is distally displaced, the arms1080, 1082 are cammed to an open position to receive the prosthesis 100,as seen in FIG. 38D. The prosthesis is mounted to the inserter such thatkeys 1076, 1078 on the inserter fit into pockets 144, 174 on theprosthesis 100. Referring also to FIG. 39, the rod is then displaced toa second position to close the arms 1080, 1082, which simultaneouslyengage with the dovetailed inner edges 142, 172 of motion stops 140,170, gripping the prosthesis securely. Each arm 1080, 1082 has anaperture 1084. The apertures may be observed with fluoroscopy to monitorthe prostheses as it is inserted into the intervertebral space, tomonitor and determine the proper depth of implantation.

The desired prosthesis is chosen and mounted on the inserter 1050. Theinserter is placed onto the retainer 800, with guiding wings 1056 overthe plates 808, 810 of the retainer. The leading (posterior) edge of theprosthesis is inserted into the prepared intervertebral space. At thispoint, the retainer 800 may be compressed slightly to facilitateendplate fixation. The inserter 1050 may be tapped with a hammer ormallet (not shown) to drive the prosthesis farther into theintervertebral space. As the prosthesis is inserted, the leadingself-cutting teeth 130 may cut a track into the vertebral endplates, andthe larger second row of teeth 131 enlarge the track. Compression anddistraction may be adjusted as needed by the retainer to ensure firmimplantation of the teeth 130, 131 into the vertebral endplates. Whenthe implant is adequately placed, the inserter handle 1052 is twisted torelease the inserter arms 1080, 1082 from the implant. The retainer 800and pins 762, 764 are removed and fluoroscopy may be used as needed toassess the final implant placement.

If needed, a tamp may be used to finely adjust the implant until it isfully seated. FIG. 40A illustrates tamp 1100, which comprises handle1102, shaft 1104 and tamp head 1106. FIG. 40B is an enlarged view of thetamp head 1106, which comprises a body 1108, a distal curved edge 1110from which protrudes a tab 1112. The body 1108 may comprise a widerupper portion, a taper and narrow lower portion. The taper and widerupper portion may act as a blocking element prevent the tamp and/orprosthesis from being pushed too far into the intervertebral space. Thecurved edge 1110 is shaped to complement the curved shapes of theanterior ends 112, 152 of the end plates 102, 104 (seen in FIG. 2). Asseen in FIG. 41, the tab 1112 is shaped to coaxially mate with thepockets 144, 174 on the end plates. The tamp 1100 may be fitted onto theanterior ends of the end plates 102, 104 with the tab 1112 in thepockets to ensure proper alignment of the end plates relative to oneanother, and a correct lateral position of the tamp. The tamp may bestruck with a hammer or mallet (not shown) to precisely seat the endplates 102, 104 in the vertebral bodies. Other embodiments may includesingle endplate tamps which are configured to seat each end plateindividually. Ensuring proper placement and alignment of the prosthesiswill allow the patient to have the optimized range of motion. After theimplant is seated in the desired position confirmed by fluoroscopy, allinstrumentation may be removed and the surgical site closed.

Each implant described herein may be revised or removed in the same or asubsequent procedure. For implant revision or removal, the patient isagain prepared in a neutral posture position, and the target disc levelof the spine is exposed from an anterior approach. Optionally, theadjustable retainer 800 and pins, or a distractor, may be used todistract the vertebral bodies. FIG. 42 illustrates a remover tool 1150gripping implant 100 prior to removal. FIG. 43 illustrates a distal endof remover tool 1150 gripping the implant 100, and FIG. 44 is anexploded view of the distal end of the remover tool 1150.

Referring to FIG. 42, remover tool 1150 comprises a first lever 1152 anda second lever 1154 joined at proximal ends by a ratchet mechanism 1156,and joined at distal ends by a rivet 1158. A pair of leaf springs 1160,1161 provide resistance as the levers are ratcheted together as theimplant is gripped. Referring to FIGS. 43 and 44, a distal end of firstlever 1152 comprises a body 1162 with a recess 1164 at the distal mostend. Two prongs 1166, 1168 enclose the recess from opposing sides suchthat the tips 1170, 1172 of the prongs oppose one another but do notmeet. The tips 1170, 1172 are angled to interface with the dovetailedinner edges 142 of anterior retention member 140 of superior endplate102 (seen in FIG. 3). The distal end of second lever 1154 comprises abody 1176 with a prying feature comprising a wedge or chisel point 1178.On an inside face of the body, a lip 1180 and a fillet 1182 extendacross the body.

The remover tool 1150 may be opened by releasing the ratchet mechanismand moving the levers 1152, 1154 apart at their proximal ends, so thatthe distal ends, rotating about the rivet 1158, also move apart. Thebody 1162 of the first lever 1152 is engaged with the implant such thatthe anterior retention member 140 on the superior end plate 102 fitsinto the recess 1164, with the prongs 1166, 1168 around the member 140and the prong tips 1170, 1172 mated, or interfaced, with the dovetailedinner edges 142. The wedge point 1178 on the second lever 1154 is wedgedbetween the superior end plate 102 and the vertebral body 2, prying themapart. Alternatively, the wedge point 1178 may be utilized before, orsimultaneously, with the engagement of the body 1162 with the superiorend plate 102. The remover tool is closed by ratcheting the leverstogether, and the bodies 1162, 1176 move toward one another, sandwichingaround the anterior end 112 of the superior end plate 102. The end plate102 is securely gripped, as the member 140 is fitted into the recess1164, and the fillet 1182 fits around the superior anterior edge of theend plate. The superior end plate 102 is pulled anteriorly out of theintervertebral space, and the nucleus and inferior end plate are pulledout along with the superior end plate, as a result of the closeoverlapping juxtaposition of the implant components. As the superior endplate 102 is pulled anteriorly, its lateral motion stops 134, 135 engagethe notches 300, 302 on the nucleus 106; the pocket 310 in the nucleusengages the post 180 on the inferior end plate 104; thus the nucleus andthe inferior end plate are pulled out along with the superior end plate.After removal of the implant, a replacement prosthesis may be implanted,or the prosthesis may be replaced with a fusion device or other system.

Alternatively, the remover tool may be turned 180° and used to grip theinferior end plate 104, with the anterior motion stop 170 of the endplate fitting into the recess 1164 of the first lever 1152, and with thewedge point 1178 inserted between the inferior end plate 104 and thevertebral body 4. In other alternative scenarios, if the nucleus is notpresent or the prosthesis has been distracted such that the componentsare no longer in close juxtaposition, each end plate 102, 104, may beremoved individually.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. For example,above are described various alternative examples of artificial discprostheses. It is appreciated that various features of theabove-described examples can be mixed and matched to form a variety ofother alternatives, each of which may have a different bearing surfaceconfiguration or preferred relative orientation according to theinvention. As such, the described embodiments are to be considered inall respects only as illustrative and not restrictive. The scope of theinvention is, therefore, indicated by the appended claims rather than bythe foregoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

1. An artificial disc replacement, comprising: a first end plate having a planar articular surface; a second end plate having a planar articular surface; a nucleus having a first side and a second side; a first interface formed between the nucleus first side and the first end plate; a second interface formed between the nucleus second side and the second end plate; and a first joint for a flexion and extension plane of motion, a second joint for anterior/posterior translation, a third joint for a lateral bending plane of motion, and a fourth joint for an axial rotation plane of motion; wherein the first and second joints are co-located at one of the first interface and the second interface, and the third and fourth joints are co-located at the other of the first interface and the second interface.
 2. The artificial disc replacement of claim 1, wherein the first and second joints are co-located at the first interface, and the third and fourth joint are co-located at the second interface.
 3. The artificial disc replacement of claim 2, wherein the nucleus first side includes a central planar portion which is in surface contact with first end plate planar articular surface when the first joint is in a low energy position for flexion and extension.
 4. The artificial disc replacement of claim 3, wherein the central planar portion of the nucleus first side is between two curvate transition portions of the nucleus first side.
 5. The artificial disc replacement of claim 4, wherein the nucleus second side includes a central planar portion which is in surface contact with second end plate planar articular surface when the third joint is in a low energy position for lateral bending.
 6. The artificial disc replacement of claim 5, wherein the central planar portion of the nucleus inferior side is between two curvate transition portions of the nucleus inferior side.
 7. The artificial disc replacement of claim 6, wherein the first end plate is a superior end plate and the second end plate is an inferior end plate, and wherein the nucleus first side is a superior side and the nucleus second side is an inferior side.
 8. The artificial disc replacement of claim 6, wherein the nucleus first side central planar portion is angled relative to the nucleus second side central planar portion, wherein the angle ranges from zero to six degrees.
 9. The artificial disc replacement of claim 2, wherein the second end plate further comprises at least one motion stop for limiting axial rotation.
 10. The artificial disc replacement of claim 9, further comprising a first motion stop protruding superiorly from the second end plate at an anterior end of the second end plate, and a second motion stop protruding superiorly from the second end plate at a posterior end of the second end plate.
 11. An artificial disc replacement, comprising: a superior end plate having a planar articular surface; an inferior end plate having a planar articular surface; a nucleus having a superior side and an inferior side; a first interface formed between the nucleus superior side and the superior end plate; a second interface formed between the nucleus inferior side and the inferior end plate; and a first joint for a flexion and extension plane of motion, a second joint for anterior/posterior translation, a third joint for a lateral bending plane of motion, and a fourth joint for an axial rotation plane of motion; wherein two of the first, second, third and fourth joints are co-located at the first interface, and at least one of the first, second, third and fourth joints is located at the second interface.
 12. The artificial disc replacement of claim 11, wherein the first joint is located at the first interface.
 13. The artificial disc replacement of claim 12, wherein the nucleus superior side includes a central planar portion which is in surface contact with the superior end plate planar articular surface when the first joint is in a low energy position for flexion and extension.
 14. The artificial disc replacement of claim 13, wherein the nucleus superior side contacts the superior end plate to form a soft stop to flexion.
 15. The artificial disc replacement of claim 14, wherein the third joint is located at the second interface, wherein the nucleus inferior side includes a central planar portion which is in surface contact with the inferior end plate articular surface when the third joint is at a low energy position for lateral bending.
 16. The artificial disc replacement of claim 15, wherein the nucleus inferior side contacts the inferior end plate to form a soft stop to lateral bending.
 17. The artificial disc replacement of claim 16, wherein the inferior end plate further comprises at least one motion stop for limiting axial rotation.
 18. The artificial disc replacement of claim 16, wherein the nucleus superior central planar portion is angled relative to the nucleus inferior central planar portion, wherein the angle ranges from zero to six degrees.
 19. The artificial disc replacement of claim 16, wherein the fourth joint is located at the second interface.
 20. The artificial disc replacement of claim 19, wherein the second joint is located at the first interface. 